Add zip subfolder to contributing.md (squash)

Now that 'Link Details' exists, it's a great place to discuss how to handle subfolders within zip files. "relative path" is unclear in the world of zip folders, as the root isn't clear either, so an example seemed prudent. Seemed better here than in the link discussion.

Squashed from https://github.com/phinze/homebrew-cask/pull/2617